The present invention relates to a method of treating substrates, in particular substrates having a hard surface such as ceramics, to render them water, oil, stain and dirt repellent. The present invention also relates to compositions for use in such a method.
Fluorinated silanes, i.e. silane compounds that have one or more fluorinated groups for rendering substrates such as glass and ceramics oil and water repellent, have been long known. Commercial products are available that comprise a silane having one or more hydrolysable groups and at least one non-hydrolysable fluorinated alkyl group. The products are used to treat ceramic or glass surfaces and are generally applied in the presence of an acid catalyst such as acetic acid. Although good repellency results are obtained with such products, there continues to be a desire to further improve them, in particular the durability and the coating efficiency.
Fluorinated polyether silanes are also known in the art. For example, EP 789050 discloses the use thereof for making composite film coatings. U.S. Pat. No. 3,646,085 teaches fluorinated polyether silanes for rendering glass or metal surfaces oil and water repellent. The surfaces are rendered oil and/or water repellent by treating them with a solution of the fluorinated polyether silane in ethanol or in 1,2,2-trichloro-1,1,2-trifluoroethane. U.S. Pat. No. 5,851,674 and WO 99/37720 disclose fluorinated silanes for providing an antisoiling coating to antireflective surfaces on substrates such as glass or plastic. It is taught to apply the fluorinated silane from a solution in an aprotic solvent. The necessary amount of water for the condensation reaction is supplied by allowing the treated substrates to cure in an environment of sufficient humidity.
U.S. Pat. No. 3,950,588 discloses the use of fluorinated polyether silanes to render ceramic surfaces such as bathroom tiles or cookware water and/or oil repellent. It is taught in this document that care should be taken that the coating composition is contained in an anhydrous state prior to application. Also, it is recommended in this document that the surface to be treated be free of water as a separate phase. The necessary water for the polycondensation reaction at the surface is derived from adsorbed water in the ceramic and/or the environment.
U.S. Pat. No. 6,127,000 (DeSimone et al.) describes a method of protecting substrates by applying a composition comprising carbon dioxide and a fluoropolyether, the fluoropolyethers optionally having anchoring groups such as carboxylic acids, esters or amides, to enhance the adsorption of the fluoropolyether to the substrate.
Although many fluorinated silane compositions are known in the art for treating substrates to render them oil and water repellent, there continues to be a desire to provide further compositions for the treatment of substrates, in particular substrates having a hard surface such as ceramics, in order to render them water and oil repellent. Desirably, such compositions and methods employing them can yield coatings that have improved properties. In particular, it would be desirable to improve the durability of the coating, including an improved abrasion resistance of the coating. The compositions can preferably be applied in an easy, convenient, and safe way and are compatible with existing manufacturing methods. Preferably, the compositions will fit easily in the manufacturing processes that are practiced to produce the substrates to be treated. The compositions preferably also avoid or minimize the use of volatile organic compounds while making efficient use of the fluorinated silane.
The present invention provides in one aspect a composition for treating a substrate comprising a homogeneous mixture of
(a) at least one fluorinated silane of the formula:
Rf1-[-Q-SiY3-xR1x]y xe2x80x83xe2x80x83(I) 
wherein Rf1 represents a monovalent or divalent perfluorinated group, optionally containing one or more ether oxygen atoms; Q represents an organic divalent linking group, R1 represents a C1-C4 alkyl group, Y represents a hydrolysable group; x is 0 or 1 and y is 1 or 2;
(b) compressed fluid carbon dioxide; and
(c) optionally an organic or inorganic acid.
It has been found that when a fluorinated silane of formula (I) is applied from a mixture that includes compressed fluid carbon dioxide, extremely thin oil- and water-repellent coatings can be provided, with efficient use of the fluorinated silane. Furthermore, the compositions spread well on the substrate to be treated with the result that uniform properties can be achieved over the whole surface of the treated substrate. Still further, the composition minimizes or eliminates the use of volatile organic compounds (VOCs), thereby reducing pollution and exposure to potentially harmful, and often flammable, solvent vapors.
As used herein the term xe2x80x9ccompressed fluidxe2x80x9d, with reference to carbon dioxide, means a supercritical fluid, a near-critical fluid, an expanded liquid or a highly compressed gas, depending on the temperature, pressure and composition. See, for example, Supercritical Fluids, Encyclopedia of Chemical Technology, 4th Edition, John Wiley and Sons, N.Y, vol. 23, pp. 453.
The compressed fluid carbon dioxide may be present in the supercritical, compressed gas, near-critical fluid, expanded liquid or liquid states and may be used to prepare the compositions of this invention. If liquid CO2 is used in the compositions or processes of this invention, the temperature is generally below about 31xc2x0 C. If compressed gaseous CO2 is used, the pressure is generally from about 20 to 75 bar (2 to 7.6 MPa). The CO2 may also be used in the supercritical state, i.e. at or above that temperature (31xc2x0 C.) at which CO2 cannot be liquefied by further increases in pressure. The thermodynamic properties of CO2 are described, for example, in McHugh and Krukonis, Supercritical Fluid Extraction, Butterworth-Heinemann, N.Y., 1994. The physical state of compressed fluid CO2 depends on the operating temperatures and pressures at which the desired end use is typically accomplished.
By the term xe2x80x98homogeneous mixturexe2x80x99 in connection with the present invention is meant that the composition is stable, i.e. no precipitation or phase separation occurs, for at least the amount of time necessary to prepare the composition and to apply it to the substrate. Generally, this means that the composition should be stable for at least one hour. The compositions suitable for use in this invention may be clear solutions as well as somewhat hazy mixtures.
Although the compositions may be prepared by combining the fluorinated silane directly with the compressed fluid carbon dioxide, it may be more convenient to prepare the compositions of the invention by diluting a concentrate comprising the fluorinated silane of formula (I). Accordingly, in a further aspect, the invention provides a method for making a composition for the treatment of a substrate, the method comprising combining a solution of a fluorinated silane of formula (I) in an organic solvent and compressed fluid carbon dioxide, and optionally an organic or inorganic acid, so as to prepare a homogeneous mixture containing between 0.01 and 5% by weight of the fluorinated silane in compressed fluid carbon dioxide. The organic solvent may comprise a fluorinated organic solvent.
In a still further aspect, the present invention also provides a method for treating a substrate, comprising the step of applying the composition of the invention as defined above to the substrate. The obtained coating on the substrate may be cured, generally at an elevated temperature of 40 to 300xc2x0 C., although elevated temperatures may not be required. The heat for curing can be supplied either through an initial preheat of substrates having sufficient heat capacity to provide the heat for curing, or through heating of coated substrates by an external heat source subsequent to coating.
The present invention further provides a composition for and a method of applying an antisoiling coating to a substrate having an antireflective surface; the method involves treating the antireflective surface with the instant coating composition. A preferred substrate is an antireflective substrate. The protective coating on an antireflective surface is relatively durable, and more resistant to contamination and easier to clean than the antireflective surface itself. The antisoiling coating is at least partially cured, i.e., solidified as by polymerizing and/or crosslinking.
The antireflective surface preferably includes a metal oxide film having one or more metal oxides, which have been preferably vacuum deposited (e.g., sputter coated). The antisoiling coating is preferably at least about 15 Angstroms thick, and preferably no greater than about 150 Angstroms thick, for a desirable balance in performance with respect to antisoiling, durability, and antireflectance. Preferably, the antireflective article has a first surface antireflectivity that is different by less than about 0.5 percentage units from that of the same article without the antisoiling coating.